The primary goal of this study is to determine in postmenopausal women whether administration of potassium bicarbonate (KHC03) will suppress the increase in rate of resorption of bone that occurs at night. A secondary goal is to determine whether daytime or evening administration of KHCO3 has greater suppressive effect. Ultimately, the goal is to determine whether administration of KHCO3 is effective in preventing and treating osteoporosis.